I see people get this confused all the time, and I want to help you out a little bit.
‘Everyday’ and ‘Every day’ do not mean the same thing, and they are not interchangeable.
Everyday is a one-word modifier. It is used as an adjective to describe a noun. For instance, She was wearing her everyday house dress.
Every day is two words; one is an adjective, and one is a noun. In this case, every is the adjective that is describing the noun day. The confusion between these two phrases is something that happens every day.
And now, for my final act, I will use these phrases in a sentence together.
Being late is an everyday occurrence for him; he is late every day.